Posted
by
timothyon Saturday March 31, 2012 @03:13PM
from the us-dollars-of-course dept.

Malvineous writes "Don't have $1500 to drop on a USRP? A Linux kernel developer has discovered that a Realtek digital TV tuner chip has an undocumented mode that turns it into a software-defined radio, with a frequency range of 64-1700MHz. The going rate for one of these USB devices can be as low as US$11. If you're unfamiliar with software-defined radio and have 20 minutes to spare, Balint Seeber has a great video introduction."

In a "normal" radio-using device you have an electronic circuit to detect or create an exact sort of signal at a particular frequency range. For example you have one sort of circuit to detect FM tpy signals and a completely different circuit to detect AM radio signals, and a TV has circuitry that transforms exact-TV-format signals into the needed picture and sound signals. The advantage of these specific electronics is that they are cheaper and use less power.

A software defined radio picks up (or transmits) radio waves basically as a graph. A digitized wave form. A software defined radio uses a CPU to examine (or create) the radio wave. This means that simply by loading in the right software you can detect (or create) absolutely any sort of signal at all. You have one circuit that can handle up AM, FM, TV, cellphone signals, wifi signals, or anything. They can also use advanced digital methods to eliminate various kinds of noise.

The downside of software defined radio is that the circuitry needs to be bigger, faster, and more power-hungry to handle fast computation.

Software defined radio has the government worried and paralyzed. The government is used to individually regulating the frequencies and power levels and signal characteristics of each kind of radio-using device. An AM/FM radio specifically does not pick up police or cell phone frequencies, and things like CBs and walkie-talkies and cellphones and baby monitors all have specific power levels and specific frequencies they can broadcast on, and they only broadcast in specific radio formats. And those limits are hard-baked into the devices by their exact circuitry. Software defined radio throws that entire idea out the window. A software defined radio is going to have some inherent power limit based on the exact hardware, and some minimum and maximum frequency range based on the hardware, but generally it can handle a very broad range from low frequency bands to high frequency bands, and they can send/detect absolutely any radio format over that entire range, and they can do it at full power. There's no way to regulate "don't detect police/cell frequencies", and no way to regulate "don't broadcast FM on what is supposed to be an AM band", and there is no way to regulate different power levels on different bands. Once you sell a software defined radio, the end user can load in any software they want.

Software defined radio is revolutionary. It is incredibly flexible. And that flexibility is exactly the "problem" for government regulators.

Software defined radio has the government worried and paralyzed. The government is used to individually regulating the frequencies and power levels and signal characteristics of each kind of radio-using device. An AM/FM radio specifically does not pick up police or cell phone frequencies, and things like CBs and walkie-talkies and cellphones and baby monitors all have specific power levels and specific frequencies they can broadcast on, and they only broadcast in specific radio formats. And those limits are hard-baked into the devices by their exact circuitry. Software defined radio throws that entire idea out the window. A software defined radio is going to have some inherent power limit based on the exact hardware, and some minimum and maximum frequency range based on the hardware, but generally it can handle a very broad range from low frequency bands to high frequency bands, and they can send/detect absolutely any radio format over that entire range, and they can do it at full power. There's no way to regulate "don't detect police/cell frequencies", and no way to regulate "don't broadcast FM on what is supposed to be an AM band", and there is no way to regulate different power levels on different bands. Once you sell a software defined radio, the end user can load in any software they want.

Anyone with the technical knowledge can do any of these tasks in hardware, for not a lot of money. There are entire libraries of books and technical articles on how to broadcast/receive on any band, even "forbidden" ones like 800MHz cellular. I remember an article in Popular Communications for a down-converter you could build to listen to 800MHz back in the early 90s that simply screwed into the BNC of your scanner in line with the antenna.

BFD.

>implying it's somehow illegal to listen to bands outside of AM/FM

What the hell are you talking about? Beyond the Cellular legislation, any and all bands are open for reception. It's your right to intercept radio waves on whatever spectrum and you don't need a license to do so. You only need a license to transmit on licensed spectrum.

>cannot regulate power levels and bands

As if they weren't able to regulate for the past 80 years?

Protip: If you are transmitting 1kw, and transmitting in a band you shouldn't be in, it won't be the feds who track you down, it will be the licensed operators who will find you and turn your ass in to the feds and they'll be happy to do so.

The only reason why Joe Trucker doesn't get turned in with his 1kw linear on CB is because he's a moving target. Anyone else sitting in his basement throwing shitty harmonics up and down the bands can be found.

Also, software defined radios are not amplifiers. You are conflating one technology with another.

1) The hard bit of any radio, especially important for transmission, is low noise band filtering, precise tuning and mixing to convert to/from AF. This all involves a lot of careful analog design and cannot be replaced by the S of SDR. Sure, the oscillator can use DDS, but that's not going to be done by wasting your computer's CPU cycles and it wouldn't make any difference anyway;

2) For analog modes, it's arguably easier to build a circuit than write decoding software. For digital modes, you're either running software on your desktop/laptop or you're running software on the radio's CPU. The only relevant questions are a) whether the modulation is documented - the answer is usually "yes" except for military; b) the keys for any encryption are available to you - if not, being able to implement an SDR makes not a hoot of difference;

3) Yes, you can do some fine DSP with a modern CPU but only an idiot thinks this is a substitute for a good antenna and (per 1) front-end.

SDR is the e-m equivalent of the "winmodem" in dial-up days: yes, you have the opportunity for a lot more versatility, but only by creating something dumb and offloading the work to a less power-efficient general purpose computer which may or may not have something better to do.

The only reason why Joe Trucker doesn't get turned in with his 1kw linear on CB is because he's a moving target

Actually, it's because the government just doesn't give a sh*t about CB radio frequencies. But given that it uses an incredibly simple modulation scheme, it can easily be traced and tracked in realtime. All you need to catch "Joe Trucker" is three antennas spaced one wavelength or more apart and you can get a fix on their position. They may be a moving target, but they're moving along a fixed path: The road. Find a guy heading northwest in the same direction as the highway and you just hop on the road a few exits up and join the flow of traffic. He'll talk again, and when he does... oh look, it's the guy 50 feet in front of you in the left lane... *flips on lights* Goodbye 1kW transmitter, goodbye trucker.

Be more concerned about frequency hopping mobile devices that use a PRNG to communicate with another device over a range of frequencies and encoding techniques... That requires a LOT more equipment to sort out where the signal is coming from. Actually, that's pretty much what the military does... o_o

What you described is the difference between an old two stages RF architecture going from the target frequency to a base band signal through an intermediate frequency and a direct conversion / zero IF RF architecture. All recent RF chips for wireless are zero IF nowadays.

But SDR usually refers to the digital processing part. Some modem implementations use custom digital logic to do the processing. A SDR approach will use a big DSP (vector DSP even) to do the processing in software. Although typically some heavy parts like the FEC and FFT (for OFDM/OFDMA) can still be done with custom hardware for better efficiency.

In any case, the dream of a purely generic hardware is still only a dream. We can have flexible software modem (if you're not too concerned about die size and power efficiency), we have also wide band radios. But in front of that you still require a RF front-end (FE), comprising filters (not to be blinded by adjacent channels in Rx, or not to kill adjacent channels in Tx) and power amplifier in the transmission side. And there's no much flexibility there. You can have wide-band PAs, but it's limited and efficiency will suffer (so burn more power, heat more than a narrow band PA). And filters are not configurable. If you want to support many bands combinations, you end up with many different filters and a switch.

So the post is too optimistic. You may be able to toy a bit with this hardware, but don't expect making anything solid (product quality) based on that. Still for hacking and the fun / learning value, why not?

Help a noob out; I'm just poking around on Wikipedia reading about SDRs and software defined antennas. These sound kind of like a magic pill to solve decentralized mesh networking. Stick an SDA on the roof, wire it up to an SDR, seek some marker signal identifying a freenet mesh node, focus in directional point-to-point comm to anyone in range who is running a compatible sda/sdr/router.

Software-defined filters can be arbitrarily good. Hardware filters are much more severely cost-limited. Sofware filters can be a-causal (assuming you are doing post-hoc processing) with characteristics impossible to build in hardware, like zero phase delay for all input frequencies. You can build 100-pole filters in software; sure you can do that in hardware, but it's going to get very expensive, large, and potentially noisy. You can build filters with frequency characteristics that *exactly* match the design parameters, not merely to within the tolerances of the components you use. Software filters *add* *no* *noise*. They have *zero* temperature drift. They have *zero* aging effects. They have essentially infinte power supply rejection. They don't suffer from interference. They can be re-programmed on the fly. They can be crazy non-linear (eg., if you watched the linked video, you can create a softare filter to detect and block whistlers only when they're happening and only affecting the frequencies where the whistlers are, removing only the whistler signal and nothing else, including the background; good luck doing that in analog). You can get close to all of these characteristics in pure analog hardware, but it is far more difficult and far more expensive than doing it digitally.

Given how inexpensive digital hardware has become, most of the assumptions that go into creating standard analog reciever and transmitter hardware need to be re-examined. On the reciever side, for example, the only reason you have an IF stage is because it's prohibitively expensive to use a non-superhet design and get the same performance... unless the signals are processed digitally.

Have you seen the amazing (and highly non-linear) filters available in Photoshop for processing images? Imagine applying the same sort of technology to radio: difficult things become easy, and radical things become possible.

Only for sufficiently small bandwidth and sufficiently small variation in signal power.

Ideally we'd plug an antenna straight into an ADC of infinite resolution to simultaneously pick up everyone from the broadcaster across the street to the QRP Siberian ham, sample at twice the highest frequency desired, and have infinitely cheap CPU cycles to deal with the 600 million samples a second which result up to VHF (good luck with UHF and beyond!).

In practice, like I said in my first post:

The hard bit of any radio, especially important for transmission, is low noise band filtering, precise tuning and mixing to convert to/from AF.

For receiver-only applications, an imprecise tune could be mitigated against with more CPU-wasting software processing.

If you wanted to go wild you could try to transmit by connecting a DAC directly to your matched antenna. Explaining why this isn't done is left as an exercise for the reader.

On the reciever side, for example, the only reason you have an IF stage is because it's prohibitively expensive to use a non-superhet design and get the same performance... unless the signals are processed digitally.

Yeah, and the reason my DAB receiver costs ten times as much and sucks up at least ten times the power of my FM broadcast receiver is because simple analog circuitry is insufficient. Meanwhile good IF filters may still allow you to reject a loud adjacent channel and crank up the gain to give a better amplitude on the signal of interest for your ADC.

Imagine applying the same sort of technology to radio: difficult things become easy, and radical things become possible.

Yes, I'm a ham with a math background and last year I had a bit of an obsession with writing DSP filters, motivated by some horrible local use of PLT. A good frontend and especially a good antenna remain your primary aims - no amount of DSP with a bad front-end will give you a comparable output and we're back to the theoretical ADC of infinite resolution to pick up those minute changes in the presence of overwhelming noise.

Moving to the digital domain for AF filtering and demodulation is fine, but an FPGA is a better option than stressing a generic CPU, making it less an "S"DR anyway.

How do you think that a modern mobile phone works? Look at the radios in them. They are all SDR: a saw filter, an I/Q mixer and an analog to digital converter. Then it goes into a DSP. There are billions of mobile phones using SDR today.